#include <iostream>
#include <array>

using namespace std;
 
int main() {
    constexpr uint64_t num = 21; 
    array<array<uint64_t, num>, num> matrix {1};
    for (int i = 0; i < matrix.size(); ++i) {
        for (int j = 0; j < matrix[0].size(); ++j) {
            if (i - 1 >= 0)
                matrix[i][j] += matrix[i - 1][j];
            if (j - 1 >= 0)
                matrix[i][j] += matrix[i][j - 1];
        }
    }
    cout << matrix[num - 1][num - 1] << endl;
    // 137846528820
}